"I'm comparing the two hemispheres in terms of ice," he says. "We understand what appears to be a paradox — why the two polar regions appear to be doing different things."

Sea ice occurs when the water in the upper layer of the ocean starts to freeze over.

According to Simmonds, three million square kilometres — nearly half the size of Australia — of Arctic sea ice has been lost since quality satellite records began in 1979.

"The Arctic ice has been significantly decreasing every month of the year," he says.

While the Arctic has been losing around 55,000 square kilometres of sea ice a year, Simmonds says the Antarctic has been gaining sea ice — at a rate of 15,000 square kilometres a year, even as continental ice shelves melt.

He and colleagues used 30 years of observations to come up with some possible explanations for this difference in polar sea ice distribution.

They used data from satellite observations of sea ice and ice shelves, along with atmospheric circulation data, and ocean salinity and temperature measurements.

Polar opposites

While the Arctic consists of a land-locked ocean surrounded by continents, says Simmonds, the Antarctic is the "diametric opposite": a continent surrounded by ocean.

He says the cold continent of Antarctica and the relatively warm surrounding ocean provides a strong "thermal contrast" at the same latitude as the prevailing westerly winds.

This provides perfect conditions for the development strong storms that push the ice further north to where it wasn't before, says Simmonds.

As storms push sea ice northwards in Antarctica, more ice is formed at the coast.

In addition to this, there a lot of ice is being lost from continental ice shelves in Antarctica.

As this ice was formed from rainwater, it adds a large amount of freshwater to the upper ocean layers.

Being less dense than salt water, this layer of freshwater prevents convection of deeper warmer water to upper layers and also encourages the growth of sea ice on the surface.

By contrast, says Simmonds, there is no continental ice shelf melt at the Arctic. In addition, the interface between the ocean and continents there is too far north to be influenced greatly by storms, and the temperature gradient between the two is not as great as in the Antarctic.

Simmonds says he and colleagues are unclear of the relative importance of the two factors in determining the amount of sea ice in polar regions.

He says one problem in investigating this hypothesis is the complexity of interactions between different factors.

"The system down there is so complex. It involves oceanography, atmosphere, sea ice and land ice. All at a time when CO2, other greenhouse gases, and the ozone hole are changing," says Simmonds. "It's like a moving feast."

However, he hopes the work will help in the effort to fine tune our understanding of the polar regions, a key component of global climate change models.